library management: "suggested library fixes"

Groove Basin should show a user interface with a list of "suggested fixes" to the user's library. The user could accept or reject these fixes. They would be things like this:
- [ ] duplicate detection and elimination
- if a song is byte for byte the same (check md5's) then ignore the new song
- use heuristics to guess if songs are probably the same (using tags). if
we are reasonably confident that the songs are the same, delete the one
with the lower quality.
- if we're not confident enough, there will be an
api that lists possible duplicates and actions to resolve them.
- [ ] update the duration metadata field with the exact correct duration
- [ ] moving songs to their canonical file path as specified by the user
- [ ] list files that cannot be played and offer to move or delete them. This includes DRM-protected files!
- [ ] delete empty folders in music library
- [ ] submit musicbrainz information for their tracks and feel good about it
- [ ] replace their own tags with musicbrainz information when possible
- [ ] write new song tags to disk
- wrap audio streams in a container format in order to be able to write tags